This application claims priority under 35 U.S.C. 119(e) to co-pending U.S. provisional application Ser. No.60/026,428, filed Sep. 20, 1996, the contents of which are hereby incorporated by reference.
This invention relates generally to interferometers. More particularly, it relates to methods and apparatus for improving the accuracy of interferometric measurements.
In optical interferometry, two optical beams are compared by combining the beams on a detector that detects the interference between the beams. Typically, coherent reference and measurement beams are directed along a constant length reference path and a measurement path, respectively. Changes in the length of the measurement path are precisely determined by detecting the interference and hence the change in optical phase between the two beams. Accordingly, interferometers have many industrial uses, such as positioning wafers for the production of integrated circuits, positioning substrates for flat panel displays, and positioning cutting tools for high precision machining.
Over the last decade or so, interferometers have played an increasingly important role in integrated circuit fabrication. Typically, an integrated circuit substrate is affixed to a movable stage to which a reflective mirror is attached. A positioning tool, such as lithographic stepper, positions the stage underneath a high performance image projection system. An interferometer is used to sense the position of the mirror, and hence the stage, to control the position of stepper.
The potential precision with which interferometers can provide such position control has been significantly enhanced, to approximately the nanometer range, by technical advances in the design of various optical components, including lasers and photosensors, and in processing electronics. However, the manufacture of smaller, more advanced circuits will likely require sub-nanometer precision. Unfortunately, several important sources of error in the interferometric measurements remain, which in practice prevent the achievement of nanometer resolution. These errors include atmospheric disturbances, such as air turbulence in the measurement path; non-linear errors due to crosstalk between the measurement path and the reference path; and cosine errors introduced by any tilts of the stage mirror. Atmospheric disturbances can be controlled along the reference path by enclosing the portion of the interferometer which includes the reference mirror in a vacuum chamber. However, enclosing the entire system in a vacuum chamber, or otherwise providing a stable and known atmospheric environment for the entire system, is expensive and problematic.
Prior art systems have also attempted to compensate for fluctuations in optical path length due to atmospheric fluctuations. For example, the change of index of refraction of air with wavelength has long been known and characterized by researchers such as Edlen. See Edlen, "The Refractive Index of Air", Metrologia, 2:71-80, (1965), herein incorporated by reference. Earnshaw has proposed the use of two wavelengths to characterize the amount of air present in the measurement beam path as a means for measuring air turbulence for surveying applications. See Earnshaw, et al., "Two-Laser Optical Distance-Measuring Instrument that Corrects for the Atmospheric Index of Refraction", Applied Optics, 11:749-754, (1972), herein incorporated by reference. However, practical difficulties with implementing a high performance interferometer at a second wavelength in addition to the conventional HeNe wavelength of 633 nm have prevented straightforward application of the two-wavelength method.
A Second Harmonic Interferometer (SHI), originally described by Hopf, et al., has also been identified as a means for measuring air turbulence. See Hopf, et al, "Second-harmonic interferometers", Optics Letters, 5:386-388, (1980), herein incorporated by reference. An SHI projects two phase-locked beams at widely separated wavelengths along the optical measurement path. The shorter wavelength beam falls behind the longer wavelength beam in phase due to the increased refractivity of air at shorter wavelengths. This phase difference is directly proportional to a correction term that can be added to a basic length measurement made with an optical measurement system such as a heterodyne interferometer. Typically, the SHI uses a first frequency doubler to generate the second beam from a first beam generated by a CW laser, and a second frequency doubler to double the first beam, after the first and second beams have both traveled the measurement path, such that the phase difference between the two beams is more readily detected. For several reasons, however, no practical implementation of an SHI system has been developed to enable measurement with nanometer accuracy of the effects of air in the path to and from a moving stage mirror.
Frequency doubling a CW laser is not efficient, and often very little second harmonic radiation is obtained, limiting the SHI system signal-to-noise ratio and hence the accuracy of the measurement. Design compromises to compensate for the low signal-to-noise ratio lead to further errors in the measurement of the correction term for air. For example, focusing lenses, placed around the frequency doubling modules to increase the doubling efficiency, introduce at least three significant sources of error into the SHI measurement. First, thermal drifts in glass elements differ for the two wavelengths. Second, angular variations in the position of the stage mirror couple with the lenses to produce errors. As the stage tilts, the beams travel through different parts of the lens aperture. Wavefront errors in the lenses due to fractional wavelength departures from a completely achromatic design introduce a phase shift between the two-wavelengths used-in the second harmonic interferometer. Finally, the lenses can amplify angular variations in the beams as they travel through the second frequency doubler. The effect of these errors is to introduce a variable phase shift between the two wavelengths which is indistinguishable from that caused by the air turbulence in the path. Averaging the data to improve the signal to noise ratio is problematic when measuring a moving stage mirror as is typical during the measurement period.
SHI systems also have unique data processing requirements for determining the phase of the SHI signal. Typically, "1/f" noise is reduced in an interferometer by modulating one of the beams to introduce a frequency shift of between about 2 and about 20 MHz between the beams. The Doppler frequency shift in the beat frequency of the beams, due to the movement of the stage mirror, is detected. However, using these techniques in an SHI typically requires that either the fundamental beam or the second harmonic beam is modulated. Practical modulators have a low efficiency, and thus a significant loss is introduced into either the very small amount of second harmonic light, or the remaining fundamental light, where the effect of the loss is squared during the second frequency doubling process. Alternatively, a phase dither can be imparted to one of the SHI beams. The data processing to analyze the effects of this dither to determine the phase difference between the two beams usually incorporates averaging techniques to compensate for the low system signal to noise ratio. Averaging the data samples creates a lag in the value of the correction due to data age, and introduces errors when the stage is moving.
Finally, a CW laser source with sufficient power to create a detectable amount of second harmonic light is typically rather large, requiring that many of the system elements be placed away from the measurement path, leading to "deadpath" errors.
Accordingly, it is an object of the present invention to provide improved apparatus and methods for reducing the error in interferometric measurements caused by the presence of an atmosphere along the measurement path of the interferometer system
It is another object of the present invention to provide an improved interferometer system for determining the effects of an atmosphere present along a measurement path.